The present invention relates to a semiconductor memory device and manufacturing method, and more particularly to a semiconductor memory device that has a tantalum pentoxide layer as a capacitor dielectric film and a method for manufacturing the same.
As semiconductor manufacturing technology has progressed and the applications of memory devices has expanded, highly integrated memory devices have been developed. One type of memory device, a dynamic random access memory (DRAM) cell, consists of a single capacitor and a single transistor and is capable of being made using large-scale integration techniques.
DRAM integration has developed by a factor of four during the last three years. 4 Mb DRAM chips are currently in mass-production, the 16 Mb DRAM is quickly being developed for mass-production, and both 64 Mb and 256 Mb DRAMs are being researched and developed.
It is crucial that these new generation semiconductor memory devices have sufficiently large capacitances in which to read and write information. However, as integration density increases, it is increasingly difficult to obtain sufficient cell capacitance as the size of each memory cell must be reduced in order to accommodate more memory cells per unit area. Accordingly, methods have been sought to achieve larger capacitances within limited areas. These methods are: (1) reducing the thickness of the dielectric film, (2) increasing of the effective area of the capacitor, and (3) using materials having a large dielectric constant.
With respect to the third method of using materials having a large dielectric content, it is necessary to use a very strong dielectric material as the capacitor's dielectric film to ensure good dielectric properties. Various oxides, e.g., tantalum pentoxide (Ta.sub.2 O.sub.5), yttrium oxide (Y.sub.2 O.sub.3), hafnium oxide (HfO.sub.2), etc. have been suggested, with tantalum pentoxide being widely considered the most preferred dielectric material due to its high dielectric constant and thermodynamic stability.
Tantalum pentoxide has a dielectric constant of about 22-25 for a thin film, and can be applied using chemical vapor deposition (CVD). As for the CVD methods, many variations have been suggested, e.g., a low-pressure chemical vapor deposition (LPCVD), a plasma-enhanced chemical vapor deposition (PECVD), a photo chemical vapor deposition (photo-CVD), etc. of these, tantalum pentoxide film formed by the PECVD method has a densely formed film composition, and results in a carbon-free film. Thus, a tantalum pentoxide film made by the PECVD method has very good electrical characteristics in comparison to such a film made using the LPCVD method.
Tantalum pentoxide films formed by the LPCVD method, though it has drawbacks of high leakage current and a low breakdown voltage, has still been widely used for three-dimensional memory cell structures having a large aspect ratio because this method exhibits good step coverage and is suitable for mass-production.
U.S. Pat. No. 4,734,340 describes a method attempting to solve the problems related above in which titanium (Ti) or titanium oxide (TiO.sub.2) is doped into the tantalum pentoxide film to improve its electrical characteristics. Manufacturing equipment is required to simultaneously supply both the raw materials of tantalum and titanium, in order to simultaneously deposit these materials to form a tantalum pentoxide film doped with titanium. Tantalum penta-ethoxide (Ta(OC.sub.2 H.sub.5).sub.5) is used as the tantalum material and a titanium tetra-isoproxide (Ti(iso-OC.sub.3 H.sub.7).sub.4) is used as the titanium material. Significant improvements are most noticeable when 1.8 wt. % of titanium, based on the amount of the tantalum, is added. When the titanium was thus doped, the leakage currents of about 5 10 .sup.9 A/cm.sup.2 in an electrical field of 1 MV/cm, were obtained. Overall this method shows consistently excellent characteristics without any electrode-dependency.
However, in order to use the method suggested by U.S. Pat. No. 4,734,340 new apparatus for supplying the titanium, in addition to the conventional equipment is required. Moreover, tightly controlling the titanium concentration (from about 1.5 to 2.5 wt. % based on the amount of the tantalum) is very difficult. Further, the process becomes significantly difficult to accomplish as the number of components increase in ordinary chemical vapor deposition methods. Additionally, the titanium oxide has a low work function of electrons, so that if a titanium oxide forms condensates in the tantalum pentoxide film, the leakage current increases along the precipitates of the titanium oxide, which deteriorates the electrical characteristics.